1. Field of the Invention
The present invention relates to methods and apparatus for connecting a tubular element such as a riser pipe to an underwater wellhead and, more particularly, to such methods and apparatus which can be used in the event of misalignment of the riser pipe and wellhead and/or in the event of wellhead internal thread damage.
2. Setting of the Invention
A well-known configuration for an offshore oil or gas well drilled from a temporary platform, such as a jack-up or semisubmersible drilling rig, is known as a mudline suspension well. In drilling such a well, a large tubular conductor is first driven into the sea floor. The conductor includes an annular shoulder formed on the radially inner surface thereof near the top of the conductor with the shoulder being a few feet above the sea floor. The well is drilled through the conductor and after which a series of casing strings are suspended from the conductor. The largest string includes a "hanger" at the top thereof which comprises a pair of annular shoulders, one on the radially inner surface and one, known as a collar, on the radially outer surface. The string is lowered into the wellbore with the collar being set on the conductor shoulder thereby suspending the string in the bore. Flow-by ports or channels in the collar permit fluid flow in the annulus between the casing string and the conductor, as is necessary during cementing and other completion operations. Additional casing hangers having similar collars and radially inner shoulders may be concentrically suspended within the first-installed casing string in a manner similar to the installation of the first hanger in the conductor.
After all of the casing hangers are in place and cemented, each of the casing strings can be extended upwardly to the deck of a platform (set over the well) by means of concentric tie-back casing strings for production of oil and gas from the well to the platform. Usually, the initial connection between an underwater wellhead and the platform is made by a string of riser pipe having a set of threads at the lower end thereof. The riser pipe string is lowered to the wellhead through guides which are fixed at various elevations within the platform. The threaded lower end of the riser is landed in the wellhead and rotated to threadably engage the riser with threads formed on the upper end of the largest casing hanger. Thereafter, a tie-back casing string for engagement with the casing string which is suspended from the largest hanger is lowered through the riser and threadably engaged with its corresponding casing string. Additional strings of tieback casing are lowered and threadably engaged with casing strings at the wellhead as described above until each of the casing strings is connected to the platform through the riser.
Several problems exist in the above-described method of making the initial riser connection to the wellhead. For a number of possible reasons, the riser may approach the wellhead with a slight lateral offset or at an angle, or both. Under such circumstances, the weight and lack of flexibility of the riser pipe may prevent the riser threads from engaging the wellhead threads. Alternatively, if engagement is attempted under such circumstances, damage to the internal threads in the wellhead can result which can prevent connecting the casing strings in the bore of the platform.
Even under circumstances when the riser is in proper lateral and angular alignment with the wellhead, if it should be lowered too rapidly, the tremendous weight of the riser can damage the wellhead threads. It should also be noted that wellhead thread damage can occur prior to the installation of the riser due to other reasons, including improper installation or removal of corrosion caps which are generally threadably engaged with the larger hanger after the well is drilled.
There exists a need for a method and apparatus for connecting a riser to a wellhead. Moreover, there exists a need for such a method and apparatus in which the threads on the lower end of the riser may be selectively oriented to facilitate alignment with the wellhead threads.
There also exists a need for making a riser connection to a wellhead after the hanger threads have been damaged.